This invention relates generally to acousto-optical devices, and more particularly, to devices known as Bragg cells, for the modulation of a light beam with an acoustic signal at radio frequencies. Acousto-optical devices have been increasingly used for the modulation of light beams in such applications as communications, electro-optical instruments, and microwave as well as sensor signal processing.
Basically, an electrical radio-frequency (rf) signal is applied to one or more electro-acoustic transducers and converted to an rf acoustical energy beam. Interaction of the acoustical beam with a light beam can be used to deflect or otherwise modulate the light beam. The theory of acoustooptical modulation has been known for some years. A review of devices employing acousto-optical principles may be found in a paper entitled "Review of Acousto-optical Deflection and Modulation Devices," by E.I. Gordon, Proceedings of the IEEE, Vol. 54, No. 10, pp. 1391-1401, October, 1966.
In general, a design goal for these acoustooptical devices is to maximize both the bandwidth and the interaction efficiency, which is the degree of modulation that can be achieved as a function of the input rf power to the device. In other words, the goal is to achieve the greatest bandwidth with the smallest power requirement. Unfortunately, however, device power consumption is proportional to the square of the bandwidth.
It is known that the product of the acousto-optical interaction efficiency and the device bandwidth can be increased by employing a phased array for the generation of the acoustic signals. In a planar phased array, multiple electro-acoustical transducers are spaced uniformly across a planar surface, and are energized by signals that are phase-displaced with respect to each other. For example each transducer may be energized by a signal that is displaced in phase by an angle .phi. with respect to an adjacent transducer. In other words the relative phase angles for successive transducers are 0, .phi., 2.phi., 3.phi., and so forth. For convenience, it is preferable to make the phase difference .phi. equal to 180.degree.. In the planar phased array, a 180.degree. phase shift between elements produces two sets of wavefronts, which result in a far-field radiation pattern with two spaced lobes. Since only one of the lobes of the far-field pattern can be usefully employed in interaction with an optical beam, at least half of the acoustical power is wasted in this arrangement. Planar arrays of this general type are discussed in a paper by A. Korpel et al. in the Proceedings of the IEEE, Vol. 54, p. 1429, 1966.
In a paper entitled "Two Hundred MHz Bandwidth Step-Array Acousto Optic Beam Deflector," by S. K. Yao et al., presented at the SPIE Symposium on Optical, Electro-Optical, Laser and Photographic Technology, August, 1976, the authors described a different type of phased array structure, in which the individual transducer elements are formed in a blazed grating, having a generally sawtooth profile. Even with phase differences of 180.degree. applied to the transducer elements, the resulting far-field pattern contains only a single lobe, and there is an accompanying improvement in efficiency.
The technique for constructing the blazed grating required that transducers be formed on a wedge-shaped piece of material acoustically identical with a body of acoustic material with which they would be used. Then the wedge-shaped piece is sliced into a plurality of wedge-shaped elements, which are then bonded to the body of material in a linear array, thereby forming the sawtooth profile. This process must be performed with extreme care and skill if the resulting array is to have the requisite precision. For microwave frequencies (above 300 MHz), it is virtually impossible to fabricate a grating using this technique, and microwave Bragg cells have had to rely on planar phased arrays, with their inherent inefficiency resulting from the generation of two acoustic beams.
Accordingly, prior to this invention acousto-optic Bragg cells operating with a one gigahertz or greater bandwidth suffered from a finite and relatively low efficiency-bandwidth product. A large rf power is required even to provide an efficiency of a few percent. For linear amplifier responses, an even larger power supply is needed. Since size, weight and power consumption are important factors in the design of Bragg cells for many applications, including space and avionics applications, there is a critical need for a Bragg cell capable of microwave operation and having a reduced rf power requirement. The present invention satisfies this need.